The present invention relates to an improved optical apparatus and in particular, to an improved optical apparatus including a main lens assembly and selectively usable supplemental corrective lenses pivotable into and out of the optical path for creating copies of originals of either the same size as the originals or one of a plurality of optical reduction or magnification ratios in response to the positioning of the main lens assembly along the optical path and/or the presence or absence of an additional, supplemental corrective lens.
Most copy machines in commercial use today are constructed so that a supplemental corrective lens element is automatically added to the optical system when the main lens assembly is at the proper positioning, changing the properties of the lens system. The mechanism for moving the main lens assembly along the optical path comprises a feed screw rod rotated by means of a motor and a nut means, on which the main lens assembly is fixedly mounted, engaged with the feed screw rod. In the apparatus described above it sometimes happens that, owing to the inertia of the driving motor, or positioning errors of position-detecting switches, or the like, the driving motor rotates excessively and then the main lens assembly overruns the position in which it should be stopped. As a result, the optical lens system creates a distorted reproduction of the original. In order to minimize the departure of the main lens assembly from the position in which it is to be stopped, it is known to use a feed screw rod of small pitch, because the smaller the pitch of the feed screw rod, the less the deviation that occurs. However, in such apparatus, it is necessary to force the screw rod to rotate a large number of rotations in order that the main lens assembly will be moved a given distance along the optical path. Consequently a long time will be required to change the magnification or reduction ratio.